Nearby minerals confirm a high-temperature origin deep within the planet.

Meteorites from Mars contain specks of graphite-like carbon trapped within them. There have been a variety of explanations for this reduced form of carbon over the years, with some even viewing it as an indication of life on the Red Planet. But new evidence indicates that the rock holding the carbon is volcanic in origin and originated from deep inside the planet, not from living organisms. This helps scientists narrow down possible chemical reactions on the planet that could create building blocks for life, the researchers say.

Scientists have found reduced carbon compounds, like polycyclic aromatic hydrocarbons, in various meteorites with a Martian origin over the past 15 years. They’re particularly interested in this type of carbon—as opposed to the oxidized carbon in carbon dioxide and carbonate minerals—because it contains carbon-carbon bonds critical to life as we know it.

There are many different ways that these compounds can form in space, so finding reduced carbon in the meteorites tells scientists very little about how it got there in the first place. Still, attempts to explain its origin abound. Using clues from the rocks themselves and the Mars environment, ideas include the possibility of living organisms, condensation from volcanic gases, to contamination once the rocks fell to Earth.

Scientists haven’t been able to pinpoint the precise location of these carbon compounds in the meteorites—until now. “That’s like suddenly having material evidence for a robbery,” says geochemist Everett Shock, of Arizona State University, who was not involved with the current study. Knowing the composition of the surrounding minerals helps scientists eliminate some possibilities for the origin of that carbon, just like forensic evidence narrows down a field of suspects for a crime.

Andrew Steele, of the Carnegie Institute of Washington, and his colleagues peered inside mineral grains in thin slices of eleven Martian meteorites using lasers. Light shining on the grains triggers emissions with patterns unique to the minerals and carbon-containing molecules inside.

The researchers found tiny clusters of graphite-like carbon, about one to ten microns wide, in 10 of the 11 meteorites. Analyzing one meteorite, Dar al Gani 476, more closely, they found pyrene, phenanthrene and other aromatic hydrocarbons mixed in with the graphite-like sheets. It’s likely that the other meteorites contain the same mixture of carbon too, Steele says.

These carbon specks were always trapped with clusters of iron, titanium, and aluminium oxide inside grains of two different minerals. These minerals indicate the carbon clusters probably solidified as molten volcanic rock cooled within the planet, the researchers say. That means the carbon in those clusters came from a source inside the planet—not a biological source.

Identifying which minerals are associated with these carbon clusters is important because the minerals provide clues to how the clusters formed, Shock says. Any future explanations for the origins of carbon on Mars will have to account for these observations, he adds.

There are several reasons to believe that this carbon did not come from a living organism, Steele says. First, Mars lacks the plate tectonics found on Earth. That means any carbon present on the surface of Mars, whether from biological life or not, would never be shoved down to the liquid mantle. And it would have had to reside in the mantle to come shooting out of a volcano mixed with molten rock later.

Also, the metal oxides encasing the carbon are among the first to solidify as the molten volcanic rock cools. That means the carbon clusters form when temperatures are around 1300 to 1400°C—far too high for life to exist nearby.

Because Steele only studied sections of rock that were a safe distance from any cracks contaminated after they fell to Earth, it’s unlikely that this carbon is due to microbes munching on the meteorite once it hit Earth.

The next question is to figure out what happens to the carbon on Mars, Steele says. The Curiosity rover, set to land on the planet in August as part of the Mars Science Laboratory mission, will analyze the elements in rocks and soil from the surface. The mobile robotic laboratory may find carbon similar to what is in these rocks, the researchers say.

But if it detects molecules completely different from what we know to be life, scientists can start to develop new theories for the formation of those molecules, knowing that the carbon came from the planet itself, Steele says.

The other means they used of reaching the conclusion is that the types of carbon found indicated a very long, slow cooling process. The kind of thing you'd expect to find in the slowly solidifying mantle of a geologically dying planet.

The C1 carbonaceous chondrites that have been been found on Earth contain the "building blocks" of life.... amino acids and other organic compounds, as well as clay minerals and water. This class of meteorites should have landed on Mars (and the Moon) as well. Why these trace volcanic graphites are deemed relevant to the search for life is unclear.

Nice work! And they covered ~ 4.2 Ga of Mars history, which should mean another test for the prediction of current mantle and core activity.

Broadlands wrote:

The C1 carbonaceous chondrites that have been been found on Earth contain the "building blocks" of life.... amino acids and other organic compounds, as well as clay minerals and water. This class of meteorites should have landed on Mars (and the Moon) as well. Why these trace volcanic graphites are deemed relevant to the search for life is unclear.

They are relevant because a) despite detecting carbon dioxide, no organics have been detected in situ on Mars, and b) if we detect organics the knowledge of the non-biological background is essential.

I think you are implying that chondrites would have supplied detectable surviving amounts of organics.

The surface seems devoid of organics, as would be predicted from the presence of UV and concommitant potent oxidizing agents.

There may be preserved organics deeper from other sources than Mars, we have yet to find out. I doubt there is much of that though. We would have seen it in these Mars meteorites as well as Moon meteorites and it seems we don't.

IIRC impactors are only delivering large amounts of organics if the atmosphere is reducing, since the impact under the early carbon dioxide dominated atmospheres of Earth or Mars creates most of them. But the same atmospheres are believed to have been mainly neutral.

Most of Earth, and it seems Mars, initial organics inventory is likely a product of the atmosphere couped to mantle processes.

My comment on the organics, water, clay minerals found in the carbonaceous chondrites was to point out that these things are potential "building blocks" for life. Trace amounts of graphite, especially of volcanic origin is not. To that extent, graphite on Mars seems as irrelevant to life on Mars as was hematite of metamorphic origin (coarse-grained, platy gray hematite).

My comment on the organics, water, clay minerals found in the carbonaceous chondrites was to point out that these things are potential "building blocks" for life. Trace amounts of graphite, especially of volcanic origin is not. To that extent, graphite on Mars seems as irrelevant to life on Mars as was hematite of metamorphic origin (coarse-grained, platy gray hematite).

My comment on the organics, water, clay minerals found in the carbonaceous chondrites was to point out that these things are potential "building blocks" for life. Trace amounts of graphite, especially of volcanic origin is not. To that extent, graphite on Mars seems as irrelevant to life on Mars as was hematite of metamorphic origin (coarse-grained, platy gray hematite).

My comment on the organics, water, clay minerals found in the carbonaceous chondrites was to point out that these things are potential "building blocks" for life. Trace amounts of graphite, especially of volcanic origin is not. To that extent, graphite on Mars seems as irrelevant to life on Mars as was hematite of metamorphic origin (coarse-grained, platy gray hematite).

YMMv if you are considering a hypothesis that is not supported by the paper under discussion. That paper found a predicted association of observing reduced organic carbon with PAH.

Btw, this seems to be an old line of research for Steele et al, which I found out since I tried to to assess what MMC was and the paper is paywalled. MMC can contain both graphite and PAH, which makes the association between its observation and observing PAH even more understandable.

The reason they go for the MMC observation is because it it is certifiable indigenous.

As I noted in my previous comment, the contents of impactors are not so much an indication on the chemical evolution potential as much as the atmosphere the impacts happens in. I wish I had time to check the refs, but IIRC the delivery mechanisms are informative as they suggest local reducing hydrothermal vent activity meant a lot for the process going from chemical to biological evolution.

As for the volatile content that went into Earth, it depends a lot on your model of the Earth-Moon impact event. Both Earth and Moon probably lost a lot of water fortunately or we would not have any dry crust. (Water contents of chondrites averages ~ 10 % AFAIK, and original Earth should have been about as wet.)

So late delivery was likely a lesser factor. Though that is from parsimony only, I don't think we can rule out some water (and then carbon) resupply.

No surprises. ARS at least offsets some of the sensationalist articles/headlines like "SCIENTISTS FIND EVIDENCE OF LIFE ON MARS!" that have appeared. I know most people think that stuff is harmless but I know people who have called me up and said "SEE! There IS life out there!" (Because I am a skeptic that life exists anywhere else).